The present invention relates to apparatus for measuring vehicle wheel speeds in accordance with digital values measured within the time duration of the cycles of an alternating voltage created by a toothed rotor of the vehicle wheel passing in proximity with a wheel speed sensor, which may be of the magnetic pick-up type.
Such wheel speed measuring apparatus is used in conjunction with anti-skid brake control systems for preventing the occurrence of an inadmissible slip of the vehicle wheel.
Presently, only the successive full cycles or only the positive or negative half cycles of the alternating voltage are used for measuring the relative time durations of the full or half cycles, since in actual practice the rotor member cannot be manufactured economically and still maintain dimensional accuracy between the teeth and the spaces between the teeth of the rotor. If a tooth and successive space were sensed at a constant wheel speed, any dimensional differences therebetween would result in a different measured value and consequently a false indication of a wheel speed change.
In avoiding this false indication of wheel speed change, however, by sensing only successive full cycles corresponding to the passing of a tooth and successive tooth space or only the positive or negative half cycles corresponding to the passing of each tooth or each space, respectively, it becomes apparent that information gaps exist, since only 50% of the duty cycle of the alternating voltage generated is used, thus limiting wheel skid control at low speeds.